Accelerated Bridge Collapse ?

The devastating failure of a newly installed pedestrian footbridge at FIU (Florida International University) has now claimed the lives of at least six people who were crushed to death in their cars as they waited at an intersection stop-light on the busy US Route 41.

Many questions are now being raised about the specific construction method used on this project known as ABC (Accelerated Bridge Construction) which is actually a proprietary specialism developed and promoted by the campus of FIU.

The most astonishing aspect is that from looking at the design renderings of this footbridge, it is clear that this was meant to be a cable-stayed bridge using a fan-array of heavy steel cables from a central steel pylon that hadn’t even been erected at the time of the collapse.

The 175 foot section which failed was only one half of the intended span. It had been constructed on site and wheeled into position only last Saturday, but was left propped up across a couple of support buttresses with none of the intended structural support from a central pylon and overhead cable-stay suspension system.

I simply do not understand how this plan ever got past any competent engineering review and public safety risk assessment ?

Re: Accelerated Bridge Collapse ?

toucana wrote:I simply do not understand how this plan ever got past any competent engineering review and public safety risk assessment ?

I think that somebody ought to ask this in some official capacity. It would seem reckless, and the consequences predictable. I think someone is likely to end up in jail on a manslaughter 2 charge, or at least they probably ought to.

Re: Accelerated Bridge Collapse ?

by wolfhnd on March 16th, 2018, 9:17 pm

I have experience with designs not being followed because someone in the construction division decided they knew better. It happens more than you would expect. The last time it happened to me they pretended they didn't know who made the decision.

I don't like cable-stayed bridges in the first place. People think they are aesthetically pleasing but as far as I'm concerned that is only the uninformed that feel that way. Aesthetics require form and function to be well integrated. A nice old fashion deep steel girder bridge is cheaper, safer, and the deck can be replaced half at a time while maintaining traffic. I have had this argument before but politics always over rules common sense.

Re: Accelerated Bridge Collapse ?

This is an unusual structural disaster. Virtually all the time it's a dilapidated structure that should have fallen down long ago but was never repaired due to infrastructure cuts.

But this was a brand new bridge. Nobody can complain about infrastructure spending. This is an engineering f*ckup of massive proportions. It will be interesting to see what shakes out of the investigation. Already there are stories about the contractors having other troubled projects. I'm sure we'll find some political shenanigans too. Somebody got paid to look the other way or somebody saved a buck cutting a corner.

Re: Accelerated Bridge Collapse ?

One point not clearly made out is that the tall mast and cable-stay system seen in the design drawings was largely ornamental in purpose. The structural strength was supposed to come from an internal steel rod and post tensioning system embedded within the concrete Warren truss members.

The primary problem was that when the ABC method was deployed to swing the prefabricated bridge section into place in a six hour lifting operation, one of the massive mobile jacks could not be positioned quite where intended because of kerbside obstructions at the north end of the span.

Even though the construction protocol specified that the mobile lifting jacks had to be located as close to the end-points as possible, the construction crew improvised by repositioning the jack at the north end further in towards the mid-point, and attempted to compensate for the changed loading by over-tightening the internal tension rod of the truss at the north end.

Disaster struck after the lift was complete, when engineers attempted to reset the tension nut at the north end to its intended value. They discovered to their surprise that the tension nut was quite loose (the rod was already in elastic failure). Two engineers were on top of the bridge trying to decide what to do next when the truss rod failed completely, and the entire bridge instantaneously collapsed.

One point not clearly made out is that the tall mast and cable-stay system seen in the design drawings was largely ornamental in purpose. The structural strength was supposed to come from an internal steel rod and post tensioning system embedded within the concrete Warren truss members.

The primary problem was that when the ABC method was deployed to swing the prefabricated bridge section into place in a six hour lifting operation, one of the massive mobile jacks could not be positioned quite where intended because of kerbside obstructions at the north end of the span.

Even though the construction protocol specified that the mobile lifting jacks had to be located as close to the end-points as possible, the construction crew improvised by repositioning the jack at the north end further in towards the mid-point, and attempted to compensate for the changed loading by over-tightening the internal tension rod of the truss at the north end.

Disaster struck after the lift was complete, when engineers attempted to reset the tension nut at the north end to its intended value. They discovered to their surprise that the tension nut was quite loose (the rod was already in elastic failure). Two engineers were on top of the bridge trying to decide what to do next when the truss rod failed completely, and the entire bridge instantaneously collapsed.

Re: Accelerated Bridge Collapse ?

by wolfhnd on April 11th, 2018, 4:45 am

If you happen to see the NTSB report please post. I originally wasn't interested because accidents during construction are almost always the result of poorly considered changes to the original plans. After reading the account it struck me that it was odd that a bridge would be designed with zero redundancy. What people are calling the faux cable stay element seems in the sketches to be a design element to add redundancy after the truss was lifted into place or at least should have been.